This invention relates to fluid pump construction and, in particular, to a multi-stage centrifugal pump.
Multi-stage fluid pumps are widely known and utilized in both commercial and residential applications. Such pumps use multiple pumping stages mounted to a rotating shaft to pump a fluid from one end of the pump to the other and to increase the fluid pressure. The pumping requirements determine the number and size of stages in the pump.
Fluid pumps involve a wide variety of design types, e.g., positive displacement, venturi and the like. One design type, a cylindrically-shaped centrifugal pump, is widely used to provide a pressurized water supply. Examples of such fluid pumps are described in U.S. Pat. No. 4,708,589 (Nielsen et al.) and U.S. Pat. No. 4,923,367 (Zimmer).
These fluid pumps comprise an inlet housing, a cartridge of stacked pump stages, and a discharge housing remote from the inlet. Each "stage" of such a pump has an impeller which "flings" water radially outward by centrifugal force; each stage also includes a diffuser assembly which encloses the impeller. The diffuser assembly may consist of one or more components. Typically, a plurality of such stages are "stacked" so that the discharge portion of one stage feeds liquid into the inlet portion of the next stage. For applications where the pump is not submerged in water, a pump casing, or shell, surrounds the cartridge of stacked stages, the inlet housing, and the discharge housing. An impeller or drive shaft, coupled to a motor shaft, extends axially through the cartridge and is fixed to the impeller within each of the stages. Rotation of the drive shaft thus turns each impeller to force fluid radially outward toward an adjacent, stationary diffuser. In turn, the cooperating diffuser directs the fluid radially inwardly and toward the next pump stage. At the inlet end of the cartridge is a motor adapter for receiving and mounting an electric motor. At the discharge end of the cartridge is an outlet from which pumped water flows.
For leakage prevention and pump efficiency, it is desirable to retain the inlet housing, the pump stages, and the discharge housing snugly compressed against one another. In one type of prior art pump, as shown by Nielsen in U.S. Pat. No. 4,708,589, compression is accomplished by a hollow, cylindrical metal shell sleeved over the stacked stages. One way compression is maintained is by crimping the shell to a motor adapter on one end and an output flange on the other. Another way is by having formed screw threads on the shell which are engageable with complementary threads on the discharge housing and the inlet housing. During assembly, the stages are slipped into the casing and the inlet and discharge housings are then threaded into the casing until they engage respective ends of the stage cartridge. Rotation may be prevented by a set screw or other fastener. The diffusers of such pumps are thus held stationary by the axial force exerted on the cartridge by the inlet and discharge housings once the latter are tightened. In another type of pump, as shown by Zimmer in U.S. Pat. No. 4,923.367, the shell is embodied as a pair of plastic half-cylinders joined together by fasteners. Compression of the stages is provided by an adjustment cone rather than by the shell.
In such prior art pumps, the axial force of the housings against the cartridge is sometimes insufficient to preclude lateral movement of the diffusers along the drive shaft because the compressive force holding the stages against each other can decrease over time. As a result, the stages (or parts of stages) may separate slightly, allowing leaks to develop. Moreover, as the axial compressive forces diminish, they may not provide sufficient frictional forces to preclude rotation of the diffuser plates and/or the diffusers (which are subjected to torsional forces by the moving water in the pulp). Both leakage and unwanted rotation of pump parts result in a decrease in pump performance.
Another disadvantage arises merely from the fact that many known pump shells are made of metal, such as stainless steel. Such shells are relatively thin walled and may dent if dropped. In a severe case, a diffuser or diffuser plate within the shell may be fractured. Fabricating stainless steel components is also a relatively expensive process. If ordinary steel, a less expensive material, is selected for the shell or for other components, rust and corrosion are inevitable. Further, for a shell of given dimensions, metal shells weigh more than those made with alternative materials such as plastic. Fabricating plastic and composites is typically a less expensive process than using materials such as stainless steel.
While most shelled pumps use metal shells, some have used glass-filled thermoplastic shells. Gay et al. teach, in U.S. Pat. No. 5,407,323, the construction of a multistage centrifugal pump with a polymeric composite (wound fiberglass) shell. An essential element of the '323 invention is that the shell is bonded directly to the pump stage diffusers in an attempt to create a water tight seal. However, in practice, it has been found that fiberglass on its own is not waterproof; water may "weep" through the channels of the fibers, resulting in leakage through the shell.